Expert testimony in the third murder trial of US student Amanda Knox says tests on the presumed murder weapon show a new DNA trace belongs to Knox, not Meredith Kercher.

The result bolsters the defence, which claims the knife was not the weapon used to kill Ms Kercher. Another piece of DNA on the knife blade initially attributed to Kercher was disputed on appeal.

Knox defence lawyer Carlo dalla Vedova told The Associated Press that the evidence shows the knife was a simple kitchen knife used by Knox. Earlier evidence showed her DNA on the handle.

The expert testified that the minute new trace showed "considerable affinity" with Knox's DNA, while not matching those of Kercher, Knox's co-defendant Raffaele Sollecito or Rudy Guede from Ivory Coast, who was convicted in a separate trial and sentenced to 16 years for the killing.

AP

Knox's former boyfriend Sollecito addressed the court, acknowledging that he hadn't taken seriously enough the accusations at the beginning because he was too caught up with his new romance with Knox to grasp what was happening.

"Me and Amanda were living the dawn of a carefree romance and we wanted to be completely isolated in our love nest," he said.

He struggled with his composure as he pleaded with the court to acquit him.

"I hope I'll have the chance to live a life, a life, because at the moment I don't have a real life," he said. "That's what I'm asking you."

The DNA trace is the last new evidence to be entered in the latest trial. Prosecutors begin their summing up later this month, followed by the defence in December. A verdict is expected in January.